1. Field of the Invention
The present invention relates to an objective lens and an image pickup device using the same and is appropriate in particular to a camera system in a device using an image pickup element such as digital still camera, a video camera, a broadcasting camera or a photograph camera with an exit pupil kept far apart from the image field while widening an angle of field devised by employing the retro-focus type.
2. Related Background Art
Recent years, a digital still camera has been spreading out as the image input device. In this digital still camera there is generally carried out a process comprising AD-converting an output signal from a solid image pickup element such as CCD into image data, subjecting them to an image compression such as JPEG and storing the compressed data in a storage medium such as flash memory. After being developed on a computer, the compressed data thus stored are displayed on a monitor or the like.
In such a digital still camera, a higher precision of photographed images and a smaller size of image pickup devices has recently become a task and the compatibility of a high resolution and a smaller size is required therewith. Especially, to attain a thin model camera with stress laid on the portability, shortening the total lens length is required in a photographing lens (objective lens). For shortening the total lens length, it is advantageous to use an objective lens having as small a number of constituent lenses as possible.
Besides, as objective lenses for implementing a wide angle of field, many retro-focus type ones composed of a front lens unit having a negative refractive power and a rear lens unit having a positive refractive power are known. Especially as retro-focus type objective lenses having a small number of constituent lenses, those of the above front lens unit comprising one negative lens are proposed.
As such retro-focus type objective lenses with a front lens unit comprising one negative lens, for example, a 3 group 3 piece configuration comprising a negative lens, a positive lens and a positive lens in sequence from the object side is disclosed in U.S. Pat. No. 5,418,649 and a 3 group 4 piece configuration comprising a negative lens, a positive lens and a positive composite lens in sequence from the object side is disclosed in another embodiment of the same U.S. Pat. No. 5,418,649. As other examples, Japanese Patent Publication No. 7-122692 (corresponding to U.S. Pat. No. 4,984,878), Japanese Patent Publication No. 5-37288, Japanese Patent Publication No. 5-20724, Japanese Patent Publication No. 61-46807, Japanese Patent Publication No. 58-7 and Japanese Patent Application Laid-Open No. 9-189856 (corresponding to U.S. Pat. No. 5,706,141), U.S. Pat. No. 4,146,304, U.S. Pat. No. 4,674,844 and Japanese Patent Publication No. 60-32165 (corresponding to U.S. Pat. No. 4,130,349) disclose a 4 group 4 piece configuration comprising: a negative lens, a positive lens, a positive lens and a positive lens in sequence from the object side; a 4 group 4 piece configuration comprising:
a negative lens, a positive lens, a negative lens and a positive lens in sequence from the object side; PA1 a 4 group 4 piece configuration comprising: PA1 a negative lens, a positive lens, a positive lens and a negative lens in sequence from the object side; PA1 a 4 group 5 piece configuration comprising: a negative lens, a positive lens, a positive lens and a positive composite lens in sequence from the object side; PA1 a 4 group 5 piece configuration comprising: PA1 a negative lens, a positive lens, a negative composite lens and a positive lens in sequence from the object side; PA1 a 4 group 5 piece configuration comprising: PA1 a negative lens, a positive lens, a positive composite lens and a negative lens in sequence from the object side; and PA1 a 5 group 5 piece configuration comprising: PA1 a negative lens, a positive lens, a negative lens, a positive lens, and a positive lens in sequence from the object side, respectively. All of them are of retro-focus model and consist of a negative front lens unit and a positive rear lens unit. PA1 a first lens unit consisting of a first lens of negative refractive power in the shape of a meniscus with a concave surface toward the image field side; PA1 a second lens unit consisting of a second lens of positive refractive power; PA1 a third lens unit consisting of a composite lens joined of a third lens of negative refractive power with a concave surface toward the object side and a fourth lens of positive refractive power with a convex surface toward the image field side and totally having negative refractive power; and PA1 a fourth lens unit consisting of a fifth lens of positive refractive power; PA1 in sequence from the object side, wherein on letting fi, f12 and f be the focal length of an i-th lens unit, the composite focal length of the first lens unit and a second lens unit and the focal length of the whole objective lens system, respectively, the conditions of EQU 0.7&lt;.vertline.f1.vertline./f&lt;3.0, EQU 0.7&lt;f4/f&lt;3.0, and EQU 0.8&lt;f12/f&lt;2.5 PA1 a front lens unit of negative refractive power consisting of a first lens of negative refractive power in the shape of a meniscus with a convex surface toward the image field side; PA1 and PA1 a rear lens unit comprising a second lens of positive refractive power nearest to the object with a convex surface directed toward the object side; in sequence from the object side, wherein PA1 focusing is carried out by moving the above front lens unit in the direction of the optical axis. PA1 a front lens unit of negative refractive power consisting of a first lens of negative refractive power in the shape of a meniscus with a concave surface directed toward the image field side; PA1 and PA1 a rear lens unit comprising a second lens of positive refractive power nearest to the object with a convex surface directed toward the object side; wherein PA1 focusing is carried out by moving the above front lens unit in the direction of the optical axis; PA1 driving means for moving the above front lens unit in focusing; PA1 and PA1 selecting means for selecting a specific mode from a plurality of modes in possession of the above image pickup device, wherein when a predetermined mode is selected, the above front lens unit is moved by the driving means to a still nearer position of than the focused position for an infinite object.
Besides, the focusing approach is divided into the pan focus approach with the pint fixed at an ultrafocal length and a method for focusing by driving the focusing lens for each release. Generally, in a digital still camera of much consumed power, it is desired to lengthen the service life of a battery and the pan focus is effective for saving the power consumption. To obtain an image of high resolution, however, focusing is preferably made so as to make the pint best according to the object distance. Especially, in the case of a macro photographing with an objective lens small in shortest object distance, those of focusing mechanism is better than those of pan focus.
As regards a retro-focus type objective lens, the total thrust method by moving the whole lens system is well known. Besides, as a method of making a partial thrust while making a total thrust and a rear focus method by thrusting a lens on the image side, Japanese Patent Publication No. 45-39875 and Japanese Patent Application Laid-Open No. 55-143517 are referred to.
Besides, as a method for the thinnest possible image pickup device in optical axis, a collapsible mount method comprising moving the objective lens toward the imaging surface side and collapsing it into the front cover of a camera when not using is well known. To minimize the drive mechanism including the actuator of lenses in the collapsible mount method, it is effective to make the focus mechanism and the collapsible mount mechanism into common use by collapsing the focusing lens in a single focus lens.
In the photographing system using a solid image pickup element, the incident angle to the receiving surface of an off-axis ray increases with a smaller distance from the image field to the exit pupil, so that a problem of shading or the like takes place. Thus, the objective lens of a image pickup system using such an image pickup element must be chosen to a telecentric optical system with an exit pupil kept sufficiently apart from the image field. To be specific, the objective lens is so arranged preferably that the off-axis chief ray incident to the optical axis at an angle corresponding to the photographing face angle reaches the image pickup element forming an imaging surface nearly in parallel with the optical axis. The wider the photographing face angle is, the stronger the refractive power required for such a refraction of an off-axis chief ray as to become in parallel with the optical axis is.
Generally, affording a negative refractive power to a group of lenses on the object side from the stop and a positive refractive power to a group of lenses on the image field side from the stop is advantageous to the reduction of an angle of an off-axis chief ray with the optical axis, i.e., to make it nearly parallel to the optical axis. Namely, in a retro-focus type objective lens, the front group of a negative refractive power is generally disposed on the object side with respect to the stop and accordingly the angle of an off-axis chief ray with the optical axis can be reduced in presence of this front group. In the positive rear group, the angle of an off-axis chief ray with the optical axis can be reduced under action of positive lenses on the image field side with respect to the stop. Since both the front lens unit and the rear lens unit have an effect of making the exit pupil apart from the image field side, the retro-focus type optical system is very effective in the case of a wide photographing face angle.
Furthermore, for making the exit pupil sufficiently apart from the image field by shortening the total lens length, it is preferable to set at least the final lens of the rear group to a positive lens. More preferably, the surface on the image field side of the lens directly in front of the final lens is set to a convex surface. In such a manner, since the action of refracting an off-axis chief ray can be shared between the surfaces of three lenses including the final lens, the refractive power of the final lens cannot be made extremely strong. Too strong a refractive power of the final lens is inadequate because of causing the barrel-shaped aberration and the astigmatism.
From these, it is concluded that to employ the retro-focus type as the whole system and to set only the first lens of the negative front group and the final lens of the positive rear group to positive lenses is effective for making the objective lens telecentric at a wider photographing face angle and in a shortened total lens length and that the surface on the image field side of the lens directly in front of the final lens is preferably convex.
Furthermore, it is necessary for a better planarity of an image field to reduce the Petzval sum to some extent. If there is no negative refractive power in the above rear group, it is necessary for the reduction of the Petzval sum to intensify the refractive power of the first lens in the front group to considerable extent. On intensifying the refractive power of the first lens like this, a very large barrel-shaped aberration occurs as a result. In general cameras including a digital still camera, a large aberration is problematic. If there is a negative refractive power in the rear lens unit, the Petzval term in the rear lens unit can be control to some extent. Since the Petzval term in the front lens unit comprising a single lens is not corrected within the lens unit, it is only necessary to set the Petzval term in the rear lens unit in such a manner as to correct the front lens unit. Thus, for better image field characteristics, it is required that the rear lens unit has at least one concave lens or a group of composite lenses having a negative refractive power.
The configurations of Japanese Patent Publication No. 5-20724 and U.S. Pat. No. 4,674,844 have a problem that the exit pupil cannot be made sufficiently apart from the image field in the case of a shortened back focus because the final lens is not a positive lens.
The 3 group 3 piece configuration of U.S. Pat. No. 5,418,649 and the configuration of Japanese Patent Publication No. 7-122692 have a problem that the exit pupil cannot be made sufficiently apart from the image field while reducing the Petzval sum because there is no concave lens in the rear group.
In the 3 group 4 piece configuration of U.S. Pat. No. 5,418,649 and the configurations of Japanese Patent Publication No. 61-46807 and U.S. Pat. No. 4,146,304, the rear lens unit has a negative lens, which constitutes a positive composite lens. This negative lens is effective in the chromatic aberration, but these configurations have a problem that the exit pupil cannot be made sufficiently apart from the image field while reducing the Petzval sum.
In Japanese Patent Publication No. 5-37288, the rear lens unit has an independent negative lens. However, this negative lens is the lens directly in front of the final lens. Since the surface on the image field is concave, the exit pupil cannot be made sufficiently apart from the image field unless the refractive power of the final lens is made considerably greater. In the configuration of Japanese Patent Publication No. 5-37288, the refractive power of the final lens is not so strong as to make the pupil sufficiently apart from the image field. Besides, on excessively intensifying the refractive power of the final lens for making the pupil sufficiently apart from the image field, a barrel-shaped aberration and a astigmatism occur and therefore this intensification is inappropriate. Also in the configuration of Japanese Patent Publication No. 58-7, the negative composite lens directly in front of the final lens is of meniscus shape having a strongly concave surface on the image field side.
Besides, in Japanese Patent Application Laid-Open No. 9-189856, the composite lens is so constructed as to have a convex surface toward the image field side but to be 48.degree. in the angle of field and does not correspond to an objective lens of a wide angle of field. Furthermore, since the refractive power of the composite system comprising the first lens unit and the second lens unit is weak, the lens back is so long that a compact camera cannot be constructed in the direction of the optical axis.
Besides, on employing the total thrust as a focusing method, the weight of the moving lens unit becomes heavy. In a camera having an autofocus mechanism, a good response of the focusing lens is desired. And, in a camera of much consumed power, a small power needed for the focusing is desired. For this purpose, the focusing lens is preferably as small in size and light in weight as possible and the total thrust is disadvantageous in this respect. Japanese Patent Publication No. 45-39875 has also a similar problem.
The rear focus scheme of Japanese Patent Application Laid-Open No. 55-143517 requires a space for thrust on the side of the focusing lens. Besides, a space for adjusting the shift of a pint due to manufacturing errors or assembling errors is further required in front of or behind the focusing lens, which is disadvantageous to constructing a compact camera in the optical axis direction. And, in an attempt to the compaction at the time of not photographing, it is required to move lenses on the object side rather than the focusing lens and a plurality of drive systems are required.
Besides, on employing the first lens unit as a focusing lens in the configuration of Japanese Patent Publication No. 60-32165, variations in short-distance spherical aberration and curvature of field are marked. Since in a short distance, the spherical aberration varies greatly on the under side and the curvature of field varies greatly on the over side, no focusible construction is formed in the first lens unit.